We have shown that MuMTV possesses two phosphoproteins, p23 and p27, and we have also developed new methods for the purification of these proteins utilizing hydrophobic chromatography. We will determine whether these proteins, as do type-C viral phosphoproteins, exhibit type-specific binding to MuMTV RNA, and whether specificity is related to secondary structure of RNA or to sequence-specific recognition. During the next year, we also hope to explore the possible biological significance of our discovery that MuMTV-producing cells in culture shed the major glycoprotein, gp47, as well as the precursor glycoprotein polyprotein, pre gp70, in large quantities as soluble proteins. The shedding of large quantities of these proteins in vivo may provide a means to complex possibly cytotoxic antibodies against the glycoproteins and thereby increase the survival advantage of the tumor cells. We will therefore quantitate the MuMTV glycoprotein-antibody complexes in the serum of mammary tumor-bearing mice as well as the anti-gp47 and anti-gp34 globulins present after removal of such complexes. Whether MuMTV-producing heterologous (non-murine) cells also shed glycoprotein and glycoprotein precursor into the medium will be determined, using feline and mink cell lines infected with and chronically producing MuMTV. We have isolated all of the six known MuMTV structural proteins of MuMTV by procedures utilizing hydrophobic chromatography. We are preparing mono-specific antisera against each of these proteins, and will standardize radioimmunoassay procedures to determine the expression of MuMTV proteins in various tissues of both high- and low-mammary-tumor-incidence mouse strains.